Olympus 300mm f/4 IS PRO M.Zuiko Digital ED Lens

From the manufacturer:

At one-third the size and weight of typical 600mm DSLR lenses, the M.Zuiko 300mm f4.0 IS PRO (600mm 35mm equivalent) is nimble enough for hand-held shooting and compact enough to carry on even the most arduous expeditions at 2.8 lbs. Industry-first 5-Axis Sync IS technology employs in-body and in-lens stabilization to harmoniously deliver six shutter speed steps of compensation\* and incredibly crisp, detailed photographs.

Full frame equivalent: 600mm